1. Field
The present invention relates generally to magnetic memory and more specifically to magnetoresistive memory elements.
2. Background and Related Art
In magnetoresistive random access memory (MRAM), data is stored by using an electric current to create a magnetic field for switching a magnetic direction of a ferromagnetic layer of a multilayer memory element (also known as a bit). An MRAM is described as a type of nonvolatile memory because a logical state of a memory element will persist even when power is removed from the memory device. Although there are other types of nonvolatile memory chips, MRAM allows rapid read and write sequences. And unlike dynamic random access memory (DRAM), MRAM does not require a constant flow of current to retain its data—thus consuming less power.
Traditional MRAM elements have a rectangular shape with tapered bit ends and a linear magnetization. In these linear elements, magnetic poles generated at bit-ends can form demagnetizing fields within the bit and stray fields outside the bit. The demagnetizing fields can create complex magnetic domains within the bit that are detrimental to switching thresholds and data retention. The stray fields can interfere with adjacent bits. Tapering the bit ends has been useful for spreading and weakening the magnetic poles; however, problems have not been eliminated.
Ring-shaped magnetoresistive elements have been proposed as a solution to the problem of demagnetizing and stray fields. The circular magnetization mode eliminates magnetic poles and the resulting demagnetizing and stray fields. However, implementations of ring-shaped elements have been limited to low impedance pseudo spin valve (PSV) or spin valve (SV) giant magnetoresistive (GMR) devices and require that a high current be passed vertically (perpendicular to the plane) through the element to generate a circumferential magnetic field that writes the bit. Because of the shape of the element or connecting traces, the vertical current may not be uniform and instead may concentrate near points of contact with connecting lines. Additionally, concept proposals show diagonally paired word lines that are used in write sequences for applying a hard-axis (radial-axis) magnetic field to the element. These diagonal lines are of different lengths—each with a different series resistance—and thus make uniform write and read current pulse generation more difficult and add complexity to the write and read decode drive electronics.